UNITED NATIONS (CNN) -- The world's population will rise from 6.5 billion to 9.1 billion by 2050, according to a United Nations survey released Thursday.

Much of the growth will take place in the least-developed countries, where a high rate of mortality is outweighed by an even higher rate of fertility. Their current collective population of 800 million is projected to swell to 1.7 billion in 2050.

"It is going to be a strain on the world, but it seems feasible," said Hania Zlotnik, director of the U.N. Population Division. "It doesn't seem that there is a crisis coming, [but] that doesn't mean that some countries are not facing a crisis."

The U.N. report predicts that nine countries will account for half of the 2.6 billion increase: India, China, Pakistan, Nigeria, the DRC, Bangladesh, Uganda, Ethiopia and the United States.

India is expected to surpass China as the world's most populous country by about 2025, Zlotnik said.

The average life expectancy of a child born in 2050 will be 75, according to the report. A child born today is expected to live, on average, until 65.

Zlotnik warned that the United Nations' predictions depended on controlling the spread of AIDS.

"We cannot emphasize enough the huge impact of this disease," Zlotnik said. "We also have to emphasize that these projections in the long term are assuming that humanity is going to have success in combating the spread of this disease, by mostly behavioral change and prevention."

The population of the most developed countries will remain virtually unchanged at 1.2 billion until 2050, the report says.

Fifty-one countries -- including Germany, Italy, Japan and Russia -- should have smaller populations in 45 years.

"Since 1990-1995, fertility decline has been the rule among most developed countries," the report says.

But the populations of many developed countries with falling fertility rates, such as the United States and the United Kingdom, will increase due to the arrival of migrants. Zlotnik said developed countries will absorb about 2.2 million migrants every year until 2050.